Radiometers such as imaging arrays have sensitivities that are limited by drift and 1/f noise. FIG. 1 shows a graph for a typical 1/f noise spectrum. The 1/f noise has a frequency spectrum (noise vs. frequency (f)) that generally follows a 1/f curve 11 and hence the name for 1/f noise. Above a knee frequency 15 the noise is generally white noise. The cause of 1/f noise is related to properties inherent in semiconductors, which are used in many applications including imaging arrays. The noise at frequencies below the knee frequency 15 causes the imaging array's output to drift in time. Therefore, it cannot be determined whether the output of a sensor in a radiometer or an imaging array is changing because the scene is changing or whether the output change is due to 1/f noise and drift, unless some step is taken to compensate or calibrate out the drift.
In mechanically scanned arrays the sensors are moved to scan an image. For example, a mechanically scanned array can be a line array of sensors. Mechanically scanning the imaging elements modulates the signals by creating a time varying element output as the element scans across a scene. This modulation shifts the image signal to a higher frequency and effectively separates the signal from the 1/f noise in frequencies below the knee frequency. One can subtract the average value of the signal across the entire scan from the scan signal and limit the drift to what occurs within that scan as disclosed by M. A. Janssen, D. Scott, M. White, M. D. Seiffert, C. R. Lawrence, K. M. Gorski, M. Dragovan, T. Gaier, K. Ganga, S. Gulkis, A. E. Lange, S. M. Levin, P. M. Lubin, P. Meinhold, A. C. S. Readhead, P. L. Richards, J. E. Ruhl, “Direct images of the CMB from space,” Astrophysical journal, 1996, pp. 15. This method has the advantage of not requiring any additional hardware; however, appreciable drift can still occur within the scan period. To ensure minimal impact of drift on the sensor performance, the image must be scanned at a rate at least four times the knee frequency, which modulates the image signal to be within the white noise spectrum of the 1/f noise. Because typical commercial sensors have knee frequencies of 1 KHz or more, this method cannot be effectively applied due to the high scan rates required.
The methods used to calibrate staring arrays (i.e. non-scanned arrays) do not depend on movement of the sensor elements; however, these methods can also be applied to scanned arrays if desired. One method uses a switch, called a Dicke switch, to modulate the image signal, as disclosed in Ulaby, Microwave Remote Sensing, Vol 1, Artech House, MA, 1981, section 6-9. Another method of modulating the image signal is to use a rotating optical blade, which is called an optical chopper, in front of the sensors. The Dicke switch and the optical chopper both modulate the input signal to move the image signal spectral energy away from the low frequency noise, thereby minimizing drift effects.
The Dicke switch must be installed in each element separately, and therefore adds significant cost to the array. Furthermore, the Dicke switch introduces losses that degrade the sensitivity of the array.
An optical chopper has the advantage of modulating all of the elements at once because it can be placed in front of all the sensors. The drawback of optical choppers is that they cannot spin at high enough rates to modulate the image signal above typical knee frequencies. In addition, optical choppers often create audible noise and also require significant space when used with large arrays. Because an optical chopper is a moving part, more maintenance is required.
Another method of drift compensation is called noise injection. In this scheme each sensor contains a noise source that is coupled into each sensor input. The noise source is switched on and off at a rate higher than the knee frequency. By taking the ratio of the output of the sensor during the on and off times, one can eliminate the output drift due to temporal gain fluctuations. This method is disclosed in Ulaby, Microwave Remote Sensing, Vol 1, Artech House, MA, 1981, section 6-12. John D. Kraus, in Radio-Telescope Receivers, McGraw Hill, NY, 1966, pages 289-290 discusses the same method for a radio telescope receiver. This method requires additional hardware to be designed into each of the sensors, adding significant cost. Furthermore, the ability to calibrate out drift is limited to the inherent stability of the noise source. Noise sources contain uncontrolled amplitude fluctuations, typically with a 1/f type of noise spectrum, and these fluctuations add additional drift to the output that cannot be compensated using the noise injection method disclosed by Ulaby and Kraus.
As discussed above, one common method for compensating for gain fluctuations is the so-called Dicke switch. By switching between an antenna and a stable reference temperature at a rate fast compared to the rate of gain fluctuations (typically a few khz), separate estimates can be formed of the scene temperature and a stable reference temperature. Subtracting the two estimates gives the difference between the scene and the reference, and any added noise voltage or small gain fluctuations will be subtracted out.
The problem with this method is that a radio frequency (RF) switch is not that easy to fabricate and an RF switch may introduce insertion loss before the LNA, which directly reduces the radiometer sensitivity. Switches are typically made from PIN diodes, a semiconductor device that is not easily fabricated on the same IC as the LNA devices (typically InP or GaAs HEMTs). The result is that PIN switches are usually made separately, so their inclusion increases the radiometer cost due to the additional RF integrated circuit (IC) assembly.
What is needed is a method for compensating out 1/f noise and drift for a radiometer without the need to fabricate and integrate an RF switch and without incurring the insertion loss caused by an RF switch. The embodiments of the present disclosure answer these and other needs.